It was the custom at Christmastime, or so I've read, for mummers in South Wales to wander the streets of their town, with one member hiding under a white sheet or horsehair sheet--a brethyn rhawn--while holding the skull of a mare up on a pike. The head would be adorned with ribbons and streamers of various colors, and the men would sing songs about the hardship of winter and the fate of mankind, namely death:

Once I was a young horse,
And in my stable gay
I had the best of everything
Of barley oats and hay.
But now I am an old horse
My course is getting small
I'm 'bliged to eat the sour grass
That grows beneath the wall.
Poor old horse, let him die
Poor old horse, let him die.
I've eaten all my oats and hay
Devoured all my straw
I can hardly move about,
Nor can my carriage draw.
With these poor weary limbs of mine
I've travelled many miles
Over hedges, bramble bushes
Gates and narrow stiles.
Poor old horse, let him die
Poor old horse, let him die.

In Welsh of course.

The people of the town would shut their doors and hide from the Mari Llwyd until the men would begin begging for food and drink. A contest of wits--called pwnco--would then ensue between the people inside and the mummers, trading rhyming insults until a winner was declared (namely by the other side stalling). The people inside would then open their doors and give the men spiced warm beer and cake, and a monetary contribution.

This practice is similar to other Christmas customs that used to take place in the British Isles, such as the Wren Boys' begging on St. Stephen's Day, or wassailing.

The custom seems to have died out in the 1930s or so, but has been revived by a few towns. Apparently, the custom is still performed at Llangynwyd, near Maesteg, on New Year's Day.

The Mari Llwyd also makes an appearance in Silver on the Tree, the final book in The Dark Is Rising Sequence. It is sent by the Dark Lord to frighten Bran Davies away from achieving the crystal sword Erias, but fails.

It is possible that the custom goes back well beyond the Middle Ages, into the Iron Age or earlier. It certainly has a pagan atmosphere to it (as do most Christmas customs). It is worth noting that the horse goddess Rhiannon's story is also tied to South Wales, and that she is also equated with the Irish horse goddess Macha, who is a goddess of war as well as birth. The horse in Ireland was a symbol of the sovereignty of the land, which was also represented as a goddess. The Mari Llwyd could be a survival of the worship of a fearsome horse goddess of war and of the land, a sort of female Mars.

[1]. The Welsh for mare isn't mari but caseg. Mari may be a borrowing of the OE mearh, horse, which was borrowed to make the Welsh word march. Moreover, the word llwyd would mutate to lwyd after a feminine noun. Hense, this must be a male horse.

It is possible that the earliest folklorists, not having that great a knowledge of Welsh, simply decided that mari meant "mare." Of course, this would probably throw off my whole theory, but that's life, isn't it?

However, the Mari was probably Mary, possibly as in the Virgin Mary. Now, that would be interesting, as then the horse is identified with the mother of God. This would again lead to the horse goddess hypothesis, which is strenghthend by Rhiannon's identification with Modron "Great Mother" or "Divine Mother", mother of Mabon ap Modron, the god of light and music, who, in his Gallic form Maponos, is equated with Apollo.